Sound translating apparatus



Nov. '11, 1941. MASSA 2,262,146

SOUND TRANSLATING APPARATUS I Filed Jan. 31, 1940 2 Sheets-Sheet 1 3nvento1:

(lite meg Nov. 11, 1941. F. MASSA SOUND TRANSLATING APPARATUS 2 Sheets-Sheet 2 Filed Jan. 31, 1940 ma M H .H I MN M $1 -H--- n 8L- n wvwmwwwl .H .H. .w w@ m wwdw %-m .HH. .MW MH Patented Nov 11, 1941 PATENT OFFICE SOUND TRAN SLATING APPARATUS Frank Massa, Audubon, N. L, assignor to Radio Corporation of America, a corporation of Dela- Application January 31, 1940, Serial No. 316,631

13 (llaims.

This invention relates to sound translating apparatus, and more particularly to microphones of the type having a high degree of directivity whereby high sensitivity is obtained over a relatively small solid angle and great attenuation to received sound occurs outside of this small angle of pickup, the present invention being an improvement upon the type of microphone disclosed in the patent to Swickard, 2,085,130.

Microphones of this type, while being highly directive, have the disadvantage of possessing a directional characteristic which varies greatly with frequency, and the primary object of my present invention is to provide an improved m1- crophone of the type described which will be free from the aforementioned defect.

More particularly, it is an object of my present invention to provide an improved directional microphone of the type set forth above in which the directional characteristic is substantially uniform for all frequencies.

Another object of my present invention is to provide an improved highly directional microphone as aforesaid which is compact in construction, easily portable, and highly efllcient in use.

In accordance with oneform of myinvention, I provide a plurality of groups of tubes of progressively varying length, each group constituting an individual or discrete microphone section;

each covering a particular band of frequencies;

and each microphone section having the same ratio between its effective or active length and the average wave length of the sound to which it responds, thereby giving a constant directional pattern for its frequency band. The output from each microphone section passes through an acoustical filter which passes only those frequencies which the particular section is designed to reproduce. After filtering, each group or section feeds a suitable transducer for converting the acoustical energy into electrical energy, and the electrical outputs are then added'and represent the total microphone output. The transducer may be common to all microphone sections or aseparatetransducer may be employed for each section, as desired. 5

According to another form of my present in-' vention, the several microphone groups are arranged in concentric circles in association with a common circular diaphragm. Preferably, the

. high frequency group or'section -feeds through a suitable acoustical filter tothe central portion of the diaphragm; the next frequency band may into an acoustical filter arranged in the form of a hollow, annular tube which is finally terminated on an annular section of the diaphragm surrounding the central portion thereof; a third 5 group of pipes feeds through another hollow, annular tube constituting'a third acoustical filter and surrounding the first-mentioned annulus, and so on. The particular advantages of this arrangement are that (l) extreme compactness is obtained, and (2) only a single microphone diaphragm is required.

In still another form of my invention, I provide a series of progressively longer pipes or tubes in association with a single diaphragm, the pipes being subdivided into several groups each of which groups is arranged symmetrically with respect to the geometric center of the diaphragm, and each group of pipes containing an acoustical filter which makes all the pipes of any one group attenuate predetermined frequency bands at substantially the same cut-off frequency; The effectively operating length of the line is determined by the length differenc between the longest pipe and the shortest active pipe at any frequency. At the lowest frequency of operation, all .of the acoustic, filters pass the sound and the effective microphone length is a maximum. As the frequency increases, the filters of either the shortest group or the longest of pipes serve to cut off these pipes; then the filters of the group of pipes of the next succeeding size out off these pipes, etc., thereby making the effective microphone length decrease with increasing frequency. The eifect of this is to keep the ratio of microphone length to the wave length of the sound picked up by the microphone approximately constant, thus obtaining approximately constant directional characteristics at all frequencies.

The novel features that I consider characteristlc of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional obiects and advantages thereof, will best be under- 4 stood from the following description of several embodiments thereof, when read in connection v with the accompanying drawings, in which- Figure l is a central sectional view of one form of microphone according to my invention.

Figure 2 is an end view of a somewhat different form of my present invention,

Figure 3 is a wiring diagram of the electrical analogue of the microphone shown in Fig. 1,

Figure 4 is a developed view of still another surround the central group of pipes and feed 5 form of my invention,

Referring more particularly to the drawings, I

wherein similar reference characters designate corresponding parts throughout, I have shown, in Fig. 1, a casing or shell I enclosing a rather large volume for a purpos presently to be set forth and housing a diaphragm 3 which constitutes the vibrating element of an electro-dynamic or other suitable transduceradapted to trans= form acoustical energy into electrical energy. The casing I is provided with a pair of openings 5 and 6 to the former of which is connected a bundle or group of open-ended tubes 9 and to the latter of which is connected a second bundle or group of tubes ii, the tubes 9 and II constituting discrete pickup units or devices. The bundle of tubes 9 has a relatively long effective line length, that is, the length diiference between the longest and shortest tubes of this group is relatively long, thereby making this group of tubes particularly responsive to the lower frequencies. On the other hand, the length of the line of the bundle of tubes ii, that is, the length difference between the longest and shortest tubes of this group, is comparatively short, so that this section of the microphone is particularly responsive to the higher frequencies.

Interposed between the diaphragm 3 and the low frequency line or section 9 is a low pass acoustic filter i3 which may comprise a cylindrical tube having a disc or partition I5 therein which divides the cylinder off into a pair of chambers Ill and Is, the disc 85 being provided with an opening 29. This arrangement provides a filter system shown in the upper half of the wiring diagram of Fig. 3 wherein Zr. corresponds to the impedance looking into the low frequency bundle of tubes 9, M1 in the inertance of the mass of air in the opening 2i, C1 is the acoustical capacitance of the air in the chamber If, C: is the acoustical capacitance of the air in the chamber I9, and Z0 is the impedance of the diaphragm 3. The magnitudes of M1, C1, and Ca depend on the magnitude of Zr. and Z0 and on the cut-off frequency required.

The high frequency bundle of tubes Ii terminates into a high pass filter leading to the microphone diaphragm 3 and comprising a cylindrical member 23 in which the air column has a capacitance C3. The cylinder 23 is provided with a Pair of openings 25 and ii in one side thereof, the masses of air in these openings having, re-

. taining two discrete sets or groups of tubes 9 spectively, inertances M2 and M3. If Zn is the impedance looking into the bundle of tubes It and p is the pressure of the sound waves arriving at the pickup ends of the tubes 9 and I i the complete arrangement has the relation shown in the wiring diagram of Fig. 3.

It is primarily to avoid the possibility of outside sound entering the openings 25 and 21 that the outer shell Or casing I is provided to form a complete enclosure around the openings 25 :and 21. The volume of an enclosed by the shell I must be large enough so that its acoustic impedance is preferably low compared to the impetiances of the openings 25 and 21 at the lowest frequencyat which the microphone must operate. Actually, the inertances M2 and M3 may be made Pipe of the shortest to resonate with the volume of air in the shell I 7 an inertance M'ss, the air volume in at the low frequency end of the microphone range, the effect being a sharp cut-off. Below this resonant cut-oil frequency, however, the mi-' crophone should preferably be electrically or mechanically attenuated in response because the high pass filter will become ineffective to prevent these lower frequencies from reaching the diaphragm 8.

In Fig. 2, I have shown another form of my invention similar to that of Fig. 1 and also conand I I. However, instead of bundling each group of tubes together as in Fig. 1, the tubes 9 may be arranged in an outer row or circle which terminates into an annular enclosure in which is mounted a partition having one or more openings corresponding to the opening 2I and providing the inertance M1. Similarly, the group of tubes it may be arranged concentrically within the group of tubes 9 and terminate into a common pipe in the side of which holes may be drilled to provide the inertances M2 and M3. The electrical analogue of this arrangement is also represented by the wiring diagram shown in Fig. 3.

Although I have thus far shown and described a simple two-section microphone, that is to say, a microphone in which the acoustical vibrations have been divided into low and high bands, it

is obvious that any number of sections may be employed between the lowest and highest frequency sections. In a microphone employing more than two discrete sections, the sections allotted to the intermediate frequency bands would preferably terminate into band pass filters each one of which would allow only those frequencies to pass for which the particular microphone section is intended.

A particularly useful form of my invention is shown in Figs. 4 and 5. In this form of my invention, there are coupled to the diaphragm ii a plurality of groups of pipes 3h, tit, 3h; 33a, 33, 33; 35., b, 35;N. Within the tubes tie, tit,

andtis are placed discs 32, each having one or more openings 34 in which the mass of air has an inertance M31. Between the discs 32 and the diaphragm 3 are the chambers 36, the air volume in each of which has a capacitance C31. The tubes 3%, 33b, and 33 are provided with discs 36 formed with apertures to in which the mass of air has an inertance M33, while the air volume in the chambers 42 between the discs w and the diaphragm 3 have acoustic capacitances Css. Similarly, each of, the tubes 35a, 35b, and 350 is provided with a disc 44 each of which is formed with an opening 46 in which the mass of air has the chambers 48 associated therewith having acoustic capacitances Cat. Each group of tubes is similarly constructed and the longest tube Ne of the longest group has a partition 50 therein formed with an opening 52 in which the mass of air has an inertance Mu with the air volume in the chamber 54 between the disc 50 and the diaphragm 3 having an acoustic capacitance Cu. If the impedances looking into the set of tubes 3h, 3|b, and Me are respectively Z3la, Zen, and Z310, the impedances looking into tubes 33a, 33b, and 330 are Z3311, Zen, and Zen, etc., and the impedance of the diaphragm I is Zo, the electrical wilTl hbe as shown in Fig. 6.

e length of the line of this micro hone is determined by the length difference betv een the length of the pipe Ne and the length of shortest effectively operatin u or set of pipes at any instant. The oper tii e f this form of microphone is as follows: At the analogue aaeaiac lowest frequency of operation, all of the acoustic filters M31, Car to MN, Cu pass the sound, and the effective microphone length is a maximum. As

the frequency increases, the filters M31, C31 first cut off the pipes 3, 3), and 3lc and the effective length of the line will be the length difierence between the pipe 33a and the pipe N0. Next, upon further increase in frequency, the filters Mas, C33 cut oil the pipes 33a, 33b, 33, etc. The eiiective microphone length is then determined by the length diiference between the pipe 359. and the pipe No. In this way, the eiiective microphone length decreases with increasing frequency and the eilect of this is to keep the ratio of microphone length to the wave length oi sound approximately constant, thereby obtaining approximately constant directional characteristics at all frequencies.

Since the various groups of pipes are cut oil progressively, it is apparent that the longest group of pipes is always operative and that the progressively shorter groups of pipes hecome proeressively operative with decreasing frequency, with the relatively longer pipes tending con:- stantly to be common to theeflective or operatiug portion of the microphone. U)? course, it is apparent that the longest tubes could he cut oil first, leaving the shortest tubes to remain common to the entire working portion oi the microphone without departina from the pre viously described mode of operation. The pipes it it, lb, and all, are preferably terminated hy the same filter constants, as clearly shown in his. s,

in order to cut them off substantially simultanccusly at the same frequency. The same is true with the pipes llt 'illb, and tile, and so on. reason for this is that I prefer to keep the active pipes symmetrlcaliy arranged over the dis hragrn so that no unbalanced lorce exists any particular ireauency whichwoulcl tend to roclr the diaphragm and create distortion. l er this reason, also, i dispose each oi pipes symmetrically about the geometric center iris oi the diaphragm clearly shown in .1 cular emrrodirnent oi r invention illustrated ins. vi and each. 5 cup oi tubes is arranged n. uroups oi? three, l". place each. tubular unit or each group 120 from the others at the same group and, place the pro ares 'vely longer tubes the center the :agm.

i the last-described of my invention, it will he noted that, the ireuuency increases, terror and fewer phase will supply sound energy to the microphone chap ragrn it, thus reducing the total actuating force driving the microphone, For the conventional "pressure actuated microphone, therefore, the output will decrease with rising. frequency. A. special type 01' microphone may be used so that the microphone output will not decrease at the higher frequency, such as, for example, a condenser oi? crystal microphone feeding a resistance approximately equal in magnitude-to the microphone impedance in the highest frequency of reproduction. Another alternative which may be used is a stillness-controlled dynamic microphone in which the normal pressure sensitivity would increase with increasing frequency.

Although I have shown and described several o. Since, in the pa and tie, etc., each individual tube may be replaced by a bundle or group of tubes, such as the bundles 9 and H of the modification shown in Fig. l, with an appropriate acoustic filter terminating each bundle. Various other changes will, no' doubt, readily suggest themselves to those skilled in the art. I, therefore, desire that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In a sound translating device, the combination of means for translating acoustical energy into electrical energy, said means including a vibratile element, and a plurality of groups of pickup devices of the line type coupled. to said element and adapted to direct acoustical energy thereto, each of said groups of devices having a difierent line length than any other of said groups whereby to be eiIective over a different predetermined frequency range to transmit energy to said element.

2. In a sound translating device, the combl nation of means for translating acoustical energy into electrical energy, said means including a vibratile element, and a plurality of groups of pickup devices of the line type coupled to said element and adapted to direct acoustical energy thereto, said groups of devices each having a progressively longer line whereby to be progressively efiective to transmit energy to said vibratile element over difierent predetermined ireouency ranges.

3. The invention set forth in claim 2 charactermed in that each or said groups of pickup devices comprises a discrete pickup unit.

ii. The invention set forth in claim 2 characterized in that said groups oi pickup devices comprise unit devices, and characterized further in that certain of said groups have units in common with certain other groups.

5. The invention set forth in claim 2 charac terlsecl by the inclusion of acoustic means associated with said pickup devices for attenuating the acoustical energy of certain predetermined frequencies.

6. The invention set forth in claim 2 characterized by the addition or" means interposed between each oi said groups of pickup devices and said vihratilc element for acoustically attenuating the supplied by said pickup devices to said element, the attenuating. means 01 each of said groups being effective over a difierent frequency band.

V. The invention set forth in claim 2 characterized in that said groups of devices comprise unit devices, characterized further in that certain of said groups have units in common with certain other groups, and characterized still further in that each or" said groups includes means for successively acoustically attenuating the energy embodiments of my inventionfl am fully aware transmitted to said element.

8. The invention set forth in claim 2 characterized in that said groups of devices comprise unit devices, characterized further in that each of said unit devices is provided with means for acoustically attenuating the energy transmitted thereby to said element, and characterized still further in that the attenuating means of all the units in any one group is eifective at substantially the same frequency.

9. The invention set forth in claim 2 characterized in that said vibratile element comprises a diaphragm, and characterized further in that said pickup devices comprise a plurality of open-ended tubes oi progressively increasing lengths arranged in predetermined groups symmetrically with respect to the geometric center 01' said diaphragm.

10. The invention set forth in claim 2 characterized in that said vibratile element comprises a diaphragm, characterized further in that said pickup devices comprise a plurality of open-ended tubes of progressively increasing lengths arranged in predetermined groups symmetrically with respect to the geometric center of said diaphragm,

and characterized still further in that said tubes diaphragm, characterized further in that said pickup devices comprise a plurality of open-ended tubes of progressively increasing lengths arranged in predetermined groups symmetrically with respect to the geometric center of said diaphragm. and characterized still further by the addition of a perforated disc in each of said tubes for acoustically attenuating the energy supplied thereby to said diaphragm, the discs of all the tubes in any one of said groups being effective at substantially the same frequency.-

12. In a sound translating device, the combination of means for translating acoustical energy into electrical energy, said means including a vibratile element, a' plurality of pickup devices displaced along a line and coupled to said element, said devices being adapted to actuate said element in accordance with the energy picked up thereby, and means for rendering said pickup devices progressively ineflfectlve to actuate said element as a function of the frequency of the energy picked up thereby.

13. In a sound translating device, the combination of means for translating acoustical energy into electrical energy, said means including a. vibratile element, a plurality of open-ended tubes of diil'erent lengths coupled to said element and constituting a line the length of which is determined by the length difierence between the longest and shortest eifectively operating tubes, and means for causing the eflective length of said line to decrease with increasing frequency of acoustical energy picked up thereby.

' FRANK 'MASSA. 

